This invention relates to a pneumatic servomotor for an assisted braking and, more particularly, of the type providing an improved assistance control.
It is quite usual to amplify the force, which a driver exerts on a brake pedal, by means of a pneumatic servomotor (also called a xe2x80x9cboosterxe2x80x9d), comprising a variable-volume front chamber, separated from a variable-volume rear chamber by a tight flexible diaphragm and by a rigid skirt driving a pneumatic piston which bears, through a push rod, on the primary piston of a tandem master cylinder of a hydraulic braking system. The front chamber, directed towards the tandem master cylinder, is hydraulically connected to a vacuum source whereas the rear chamber, in the opposite direction to the front chamber, is hydraulically connected, using a valve control, to a propellant fluid source, typically atmospheric-pressure air.
At rest, that is when the driver does not actuate the brake pedal, the front and rear chambers are interconnected, while the rear chamber is isolated from the atmospheric pressure. On braking, first the front chamber is isolated from the rear chamber and then air is admitted into the rear chamber.
Unfortunately, when the brake pedal is depressed, the sudden opening of the air-supply port of the rear chamber causes the pneumatic piston to move forward too fast. The initial resistance to the travel of this pneumatic piston is quite weak. As a matter of fact, on the one hand, there is a great pressure difference between the front and the rear chambers of the servomotor and, on the other hand, the operational clearances of the tandem master cylinder are taken up. This swift advance of the pneumatic piston may cyclically open the passage connecting up the front chamber and the rear chamber, which generates a noise resembling that of wipers set in motion on a dry windscreen and therefore called a xe2x80x9cwiper noisexe2x80x9d in the art. Such noise, although normal, is considered quite unpleasant by the users who fear some dysfunctioning of the system. Besides, the reopening of the passage connecting up the front and rear chambers of the servomotor reduces the assistance efficiency at the beginning of the braking process.
Therefore, it is an object of the present invention to provide a servomotor for an assisted braking, ensuring a noiseless running, especially by a reduction of the so-called xe2x80x9cwiper noisexe2x80x9d to the minimum.
Another object of this invention consists in providing a, pneumatic servomotor for an assisted braking with a quite high efficiency.
Still another object of this invention consists in providing a servomotor for an assisted braking with a quite high efficiency.
It is also an object of the present invention to provide a servomotor for an assisted braking, ensuring a quick-acting assistance.
It is another object of this invention to provide a braking system including a pneumatic assistance servomotor and a tandem master cylinder, equipped with superassistance means for emergency braking situations, of the types described in EP 0 662 894 and FR 2 751 602, and having a low variation of the starting threshold of superassistance means in the case of an emergency braking situation.
These objects are achieved, in accordance with this invention, by a pneumatic servomotor for an assisted braking, fitted with means capable of resisting too fast a forward travel of the pneumatic piston when the brake is actuated, which otherwise might lead to an undesired reopening of the passage connecting the front chamber with the rear chamber of the assistance servomotor.
In a first embodiment, the servomotor according to the present invention comprises resilient means, preferably nonlinear ones, opposing the forward travel of the pneumatic piston until the latter exerts a predetermined force.
According to a second embodiment, a device slows down the pneumatic piston by friction.
However, these first two embodiments of the device according to the invention reduce the assistance servomotor performance.
The preferred embodiment of an assistance servomotor in accordance with the present invention comprises means for a temporary isolation of part of the diaphragm, when the air-supply valve of the rear chamber is opened, such means being situated in said rear chamber, so as to temporarily limit the rear chamber area on which the pressure is exerted for the forward travel of the pneumatic piston. The forward motion of the piston allows the air to access the whole diaphragm surface area, for the desired assisted braking. Advantageously, the area of the rear chamber, which is supplied with air at first, corresponds to the axially central area of the rear chamber. In a advantageous manner too, the element affording the air confinement to the initially air-supplied area of the rear chamber is a lip, judiciously arranged coaxially with the control rod and the push rod of the assistance servomotor.
The main subject of this invention is a pneumatic servomotor for an assisted braking, comprising a first variable-volume chamber and a second variable-volume chamber, tight sealing means adapted for isolating, on actuation, the first chamber from the second chamber, means for creating a pressure difference between said first chamber and said second chamber, and a pneumatic piston capable of being driven by a difference in the pressures prevailing in said first chamber and said second chamber, characterised in that it further comprises means for slowing down the initial travel of the pneumatic piston at the beginning of the braking operation, without interfering with the efficiency of the subsequent assisted braking.
The present invention also deals with a servomotor, characterised in that it further comprises a control rod, capable of being driven by the travel of a brake pedal, said control rod bearing a first flap, constituting said means for isolating, on actuation, the first chamber from the second chamber, as well as a second flap for supplying one of said first chamber and said second chamber with a pressurized assistance fluid, said flaps cooperating with elements of said pneumatic piston in such a way that the application of the control rod onto the pneumatic piston brings about the closing of the tight sealing flap for isolating said first chamber and second chamber, and in that said means for slowing down the initial travel of the pneumatic piston, at the beginning of the braking operation, keep the pneumatic piston from moving forward faster that the control rod in order to prevent, or at least minimize, the reopening of said first tight sealing flap isolating the first chamber from the second chamber.
Another subject matter still of the present invention is a servomotor, characterised in that the means for slowing down the initial travel of the pneumatic piston comprise means for reducing the initial driving force for the pneumatic piston.
According to another aspect of the invention, a servomotor is characterised in that the means for slowing down the initial travel of the pneumatic piston comprise means for confining the pressurized assistance fluid to an area in one of said first or second chambers, so as to reduce the surface area on which the pressure difference between both chambers is initially applied.
Another subject matter still of the present invention is a servomotor, characterised in that the means for slowing down the initial travel of the pneumatic piston comprise a lip which defines, at the beginning of the braking operation, a radially-inner initial thrust chamber.
In accordance with another aspect of the present invention, a servomotor is characterised in that said lip is a lip providing a one-way isolation permitting, towards the end of the braking operation, the discharge of the assistance fluid from said both chambers.
This invention also deals with a servomotor, characterised in that the lip comprises a first area, inclined with respect to a servomotor axis and directed towards said axis, and a second area substantially perpendicular to said axis.
Another aspect of the present invention lies in a servomotor, characterised in that it comprises a flexible diaphragm, separating said first chamber from said second chamber, and in that said lip is integral with said diaphragm.
Another subject matter still of this invention is a servomotor, characterised in that the lip is connected to a radially-inner crimping base of the diaphragm.
In accordance with another feature of the invention, a servomotor is characterised in that, at rest, the lip bears on a wall of one of said chambers.
Other features and advantages of the present invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.